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Green obsidian is a naturally occurring volcanic glass characterized by its amorphous structure and typically dark green to olive-green coloration. It is a rapidly cooled felsic lava, meaning it lacks a crystalline structure. Its distinctive color, which can range from translucent emerald green to opaque forest green, is usually attributed to the presence of ferrous iron (Fe2+) or other trace elements, or minute mineral inclusions. It exhibits a characteristic conchoidal fracture, producing sharp, curved surfaces. Like all obsidian, it is metastable and can devitrify over geological time, forming fine-grained crystalline aggregates.
How to Identify
- Color
- Ranges from dark green, olive green, to sometimes a translucent emerald green. The color is often uniform but can show subtle banding.
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Smooth, homogeneous, and glassy. May contain microscopic bubbles or flow bands.
- Crystal Form
- Amorphous; lacks crystal structure. It is a natural glass.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- Found in areas of recent or geologically young silicic volcanic activity, typically associated with rhyolitic lava flows, domes, or volcanic necks. It forms where lava cools very rapidly, preventing crystal growth.
Key Facts
- Hardness: 5 to 5.5 on the Mohs scale.
- Specific Gravity: 2.35 to 2.60 g/cm³.
- Crystal System: Amorphous (non-crystalline).
- Color: Dark green, olive green, emerald green.
- Luster: Vitreous (glassy).
- Transparency: Translucent to opaque, sometimes transparent in thin sections.
- Fracture: Conchoidal (shell-like, producing very sharp edges).
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2), typically 70-75% by weight, with minor amounts of aluminum oxide (Al2O3), sodium oxide (Na2O), potassium oxide (K2O), iron oxides (FeO, Fe2O3), and magnesium oxide (MgO). The green color is often due to trace ferrous iron (Fe2+) or microscopic mineral inclusions.
Quick Check
- Color: Dark green to olive green, sometimes translucent emerald green.
- Luster: Vitreous (glassy).
- Streak: White (as it is a glass, it will scratch the streak plate rather than leave a colored powder).
Physical Characteristics
- Crystal Habit: Massive, amorphous.
- Cleavage Type: None.
- Fracture Type: Conchoidal, producing extremely sharp edges.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
Formation
Green obsidian forms from the rapid cooling of felsic (rhyolitic) lava, typically with a silica content exceeding 70% by weight. The rapid quenching prevents the formation of crystalline structures, resulting in an amorphous glass. The green coloration is usually due to the presence of trace amounts of iron (Fe2+) or other transition metals, or microscopic inclusions of minerals like chlorite or serpentine, rather than being an intrinsic property of the glass itself.
Usage
Historically, obsidian was used for cutting tools, weapons (arrowheads, spear points), and ceremonial objects due to its conchoidal fracture producing extremely sharp edges. Modern uses include lapidary work (cabochons, carvings), decorative objects, and occasionally as a surgical scalpel material due to its superior edge sharpness compared to steel. Green obsidian is particularly prized for its aesthetic appeal in jewelry and ornamental pieces.
Age Distribution
Typically Cenozoic, but can form whenever silicic volcanism occurs, ranging from millions of years old to geologically recent.
Where to Find
Oregon, USA
Known for various obsidian types, including some green varieties, particularly in areas like Glass Buttes.
California, USA
Localities such as Glass Mountain and other volcanic regions can yield green obsidian.
Mexico
Certain volcanic regions in Mexico are known to produce various colors of obsidian, including green.
Indonesia
Some volcanic islands may host deposits of green obsidian.
Ethiopia
Volcanic areas in the East African Rift Valley can contain obsidian, with green varieties possible.
Finding Tips
Look in Volcanic Regions
Focus your search on areas with recent or geologically young rhyolitic volcanic activity. Obsidian forms from rapidly cooled lava.
Identify Lava Flows and Domes
Obsidian is typically found at the margins of lava flows, in volcanic domes, or as fragments in pyroclastic deposits near silicic volcanoes.
Check for Glassy Texture
Obsidian's distinct glassy luster and conchoidal fracture are key identifiers. Look for sharp, curved edges.
Examine Color and Translucency
While black is most common, hold pieces up to light to check for green hues, especially in thinner fragments or edges. Some green obsidian can be quite dark and only reveal its color when backlit.
Safety Precautions
Obsidian fragments are extremely sharp. Always wear sturdy gloves and eye protection when handling or collecting. Be aware of unstable terrain in volcanic areas.
Similar Rocks
Black Obsidian
Obsidian
Also known as: Volcanic Glass
Mahogany Obsidian
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Snowflake Obsidian
Obsidian (Snowflake Variety)
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Andesite Glass
Andesitic Glass
Also known as: Andesitic Obsidian
Pitchstone
Pitchstone
Also known as: Hydrated Volcanic Glass
Scientific Classification
- Mineral Class
- Not a true mineral (it is a rock/mineraloid)
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- Predominantly SiO2 (70-75%), with varying amounts of Al2O3, Na2O, K2O, FeO, Fe2O3, MgO, and H2O.
- Composition
- Silicic volcanic glass, typically rhyolitic in composition.
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